1. Field of the Invention
The present invention relates to an electric power steering apparatus that drives a motor so as to reduce steering torque, on a steering wheel, that is exerted by a driver.
2. Description of the Related Art
To date, as a unit for ensuring the safety in the case where a microcomputer of an electric power steering apparatus fails, there has been proposed an interlock unit that determines whether or not the direction of steering torque and the direction of motor driving coincide with each other, and inhibits the motor from being driven in the case where the direction of steering torque and the direction of motor driving do not coincide with each other (e.g., refer to Japanese Patent No. 2891069).
In such a conventional apparatus as described above, steering torque produced by the steering of a driver is detected; a target motor current is calculated based on a steering torque signal corresponding to the detected steering torque; and a motor is driven based on the target motor current so that the steering torque exerted by the driver is reduced. Meanwhile, in the interlock unit, a drive inhibition region is calculated for the steering torque signal; in the case where the steering torque signal indicates the right direction or the neutral direction, a left-direction drive is inhibited, and in the case where the steering torque signal indicates the left direction or the neutral direction, a right-direction drive is inhibited; thus, the motor is driven in such a way as to reduce steering torque exerted by the driver, but the motor is inhibited from being driven in such a way as to increase steering torque exerted by the driver.
However, in the foregoing conventional apparatus, in some cases, the steering torque signal utilized for calculating the target motor current and the steering torque signal utilized in the interlock unit differ from each other; therefore, in the case where the steering torque signal fluctuates in a short cycle period, the interlock unit determines that the target motor current is within the drive inhibition region, even though the target motor current is correctly calculated, and thereby the drive of the motor is interrupted. As a result, there has been a problem that the stability of the steering system cannot be ensured and setting of desired steering feeling cannot be implemented.
Additionally, there has been a problem that, in the case where the setting range for the target motor current is limited in such a way that the difference between the steering torque signal utilized for calculating the target motor current and the steering torque signal utilized in the interlock unit is smaller than a predetermined value, setting of desired steering feeling cannot be performed.